From our own experience and that of our customers, the best solution if you are driving to Gibraltar is to leave your car in an underground parking in the border-town of La Linea de la Concepción. To get there, drive past the La Linea bus station and behind it you will see an underground car parking. This is the Fo-Cona parking, with the La Linea main parking being in Plaza de la Constitucion.
From there you can walk to the frontier. This is an easier option than waiting in what can sometimes be long queues coming into Gibraltar. When you get there you your vehicle will be waiting to take you on your tour of Gibraltar.
The Santa Barbara car parking is not recommended as you can get caught up in the border queue itself, which can lead to long delays. There are also other parking possibilities in the area if you require them.